April 2005

Atlanta's City Hall is home to the usual bustle of City Council meetings, bill payments, permit applications—all the things that keep a major city running. But for more than a year now, a greenroof garden of hardy sedums and herbs, along with a few annuals, has provided a respite for the building's employees and visitors.

The popular fifth-floor rooftop garden was the brainchild of City Environmental Manager Ben Taube, who thought it would serve as a nice demonstration project that would encourage other building owners to consider greenroof possibilities. Greenroofs provide numerous benefits, including energy conservation, heat absorption and stormwater mitigation.

The roof has attracted numerous visitors to City Hall, including architecture students from Clemson University and representatives of the Urban Land Institute's Sustainability Committee. "Everyone seems to be impressed," Taube says.

The roof was sponsored by the Department of Watershed Management, which currently is in the midst of a $3.2 billion overhaul of its water and wastewater infrastructure to ensure its customers clean, safe drinking water well into the future. Greenway acquisition and the greenroof are part of that commitment.

"People sometimes don't see the connection between clean water and greenspace," notes Rob Hunter, commissioner of the Department of Watershed Management, who, along with Atlanta Mayor Shirley Franklin helped dedicate the project 15 months ago. "But the more greenspace you have, the better able the system is to filter pollutants in stormwater."

The benefits of a greenroof are especially important in an urban setting, because research has shown that cities are hotter than undeveloped areas. Expanses of concrete and asphalt-shingled roofs, combined with a lack of open greenspace, produce a phenomenon that has been documented by everyone from local meteorologists to scientists at NASA. Called the "Heat Island Effect," it raises urban temperatures to much higher daily levels than those enjoyed in less developed areas.

The phenomenon, along with urban ills like degraded air and water quality, can be mitigated by building gardens and protecting greenspace. The problem is that in highly developed urban areas, there is often nowhere to put those gardens.

"Greenroof technology is a wonderful tool for mitigating the urban heat island effect and for dealing with stormwater problems," Hunter says. "Plus, it looks nice."

Bill Brigham and Ben Taube put together the team that created Atlanta's municipal greenroof. Though similar projects have been completed in Chicago, Seattle and Portland, Ore., Atlanta's greenroof project is the first such undertaking in the Southeast.

City Hall provided Atlanta officials with a solution to the lack of natural space in the downtown area. The fifth-floor roof of the newest part of the building offered enough space to plant more than 2,000 square feet of vegetation. The greenroof debuted after more than two years of research and planning. It is expected to generate reliable technical data in areas such as temperature reduction, energy efficiency, and stormwater retention.

City officials are monitoring its effect on roof life and hope to determine which plants work best in the shallow soil of a greenroof. Finally, and most importantly, they hope to show Atlanta's business community a working greenroof that can serve as a model for similar projects.

The City of Atlanta's greenroof contains 35 varieties of vegetation (mostly hardy perennials), and was designed as an "extensive greenroof" which features low soil depths and low-growing vegetation. Spearheaded by Taube and Bill Brigham, the City's landscape architect, the project was completed with the assistance of more than 10 companies and vendors who donated plants and other products and services. The City sited the greenroof outside the building's fifth-floor cafeteria and not atop the main roof so it could serve as a patio.

The greenroof idea is important because Atlanta produces enormous amounts of ground-level ozone every summer and has been designated a "severe" non-attainment area by the Environmental Protection Agency. Last year, the City exceeded federal ozone standards on 11 days, two fewer than 2003's total.

In 2001, Atlanta joined with the Georgia Environmental Protection Division and other partners in a NASA-funded project to develop and validate an improved urban air quality modeling system using high-resolution remote sensing data. The project concludes this year with what the City hopes will be a system that allows stakeholders to quantify the impact of land cover and land use changes on air quality, particularly ground-level ozone.

Taube and Brigham also expect the greenroof to play a significant role in mitigating stormwater problems by absorbing water that normally would wash into the sewer system. Additionally, they hope that eventually the City will create an incentive program—like those in other cities—for commercial, residential and industrial greenroof projects. Such incentives will become increasingly popular, since the City is working on creating a stormwater utility that will base future fees on impervious surface calculations. Traditional roofs represent large impervious surfaces, so greenroofs can be significant factors in offsetting future fees.

Yucca plants provide a splash of color among sedums that turn numerous shades of red in the City's mild winters. Plants are chosen for their hardiness and their shallow root systems.

The project represented a relatively minor expenditure, primarily because, with outside bidding starting at $163,000, the City focused on in-house program management, construction and maintenance. The Department of Watershed Management supplied $42,000 and a state grant matched by the department accounted for the remainder of the $60,000 budget. The City's Parks and Recreation Department will provide what little maintenance the roof is expected to require.

The greenroof was constructed in four phases over a six-week period. After a structural analysis concluded that the roof could support "extensive" soil (the lighter of the two types of soil used in greenroof applications), workers removed concrete ramps and asphalt, replacing it with waterproofing material donated by Teaneck, N.J.-based KEMCO Kemper Systems. The construction team then plugged existing drains and flooded the roof's surface to ensure that there were no leaks.

"It's a pleasant place to be in the spring and summer," Taube says. "We're hoping other people see it and say, 'Hey, I want one of those,' but we also want them to realize the many environmental benefits offered by greenroof systems."